Polymeric membrane based separation processes such as reverse osmosis, pervaporation and perstraction are conventional. In the pervaporation process, a desired feed component, e.g., an aromatic component, of a mixed liquid feed is preferentially absorbed by the membrane. The membrane is exposed at one side to a stream comprised of a mixture of liquid feeds and a vacuum is applied to the membrane at the opposite side so that the adsorbed component migrates through the membrane and is removed as a vapor from the opposite side of the membrane via a solution-diffusion mechanism. A concentration gradient driving force is therefore established to selectively pass the desired components through the membrane from its upstream side to its downstream side.
The perstraction process is utilized to separate a liquid stream into separate products. In this process, the driving mechanism for the separation of the stream into separate products is provided by a concentration gradient across the membrane. Certain components of the fluid will preferentially migrate across the membrane because of the physical and compositional properties of both the membrane and the process fluid, and will be collected on the other side of the membrane as a permeate. Other components of the process fluid will not preferentially migrate across the membrane and will be swept away from the membrane area as a retentate stream. Due to the pressure mechanism of the perstraction separation, it is not necessary that the permeate be extracted in the vapor phase. Therefore, no vacuum is required on the downstream (permeate) side of the membrane and the permeate emerges from the downstream side of the membrane in the liquid phase.
The economic basis for performing such separations is that the two products achieved through this separation process (i.e., retentate and permeate) have a refined value greater than the value of the unseparated the feedstream. Membrane technology based separations can provide a cost effective processing alternative for performing the product separation of such feedstreams. Conventional separation processes such as distillation and solvent extraction can be costly and energy intensive to install and operate in comparison with membrane process alternatives. These conventional based processes require substantial amounts of engineering, hardware and construction costs to install and then may require relatively high levels of operational and maintenance personnel and costs to maintain the facility in an operating status. Additionally, most of these processes require significant heating of the process streams in order to separate different components during the processing steps. This results in higher energy costs than are generally utilized by low-energy membrane separation processes.
In general, the membrane technology in the present art has the benefit of lower per unit energy costs per volume of separation than the conventional technologies in present art. However, a major obstacle in perfecting the commercial operation of membrane separation technologies is to improve the flux and selectivity of the current membrane systems in order to make the construction costs and capacity of membrane technologies economically viable on a refinery scale operation.
A myriad of polymeric membrane compositions have been developed over the years. Such compositions include polyurea/urethane membranes (U.S. Pat. No. 4,914,064); polyurethane imide membranes (U.S. Pat. No. 4,929,358); polyester imide copolymer membranes (U.S. Pat. No. 4,946,594); and diepoxyoctane crosslinked/esterified polyimide/polyadipate copolymer (diepoxyoctane PEI) membranes (U.S. Pat. No. 5,550,199).
The membranes and membrane assemblies of the prior art have been used in a configuration where a single cast layer of polymeric membrane material layer is cast either on glass, a layer of a polymer film casting substrate such as PTFE (polytetrafluoroethylene), or suitable fibrous materials that are used to facilitate the casting process and equipment. The type of casting substrate chosen affects the final membrane performance since the use of these membranes in a process application requires that the permeate be able to pass through the casting substrate as well as the cast polymeric membrane. The casting substrates of the prior art have been primarily utilized to provide a porous support which can provide the necessary mechanical strength upon which to the cast the polymeric membrane material during fabrication, in particular, when utilizing automated or semi-automated commercial fabrication equipment. It has therefore been a practice of the prior art to select the casting substrates mainly for maximizing mechanical strength while minimizing interference with the cast polymeric membrane's separation capabilities.
It is known by those of ordinary skill in the art that for a given cast polymeric membrane material, the change in the selectivity is generally independent of the thickness of the membrane and the flux is generally inversely proportional to the thickness of the membrane. Thus the direction of research in the art has been to find better membrane materials in order to achieve an improvement in the membrane selectivity and to improve the casting and mounting techniques of polymer membranes to allow thinner operational castings in order to achieve an improvement in membrane flux.
The current art has modified these single casting layer membranes by varying membrane parameters such as the membrane composition, fabrication and curing processes, and process uses. However, a characteristic of these supported and unsupported single layer polymeric membranes that are utilized for hydrocarbon service aromatic/non-aromatic separations is that the aromatics in the permeate have a similar carbon weight distribution as the aromatics in the feed. That is to say, that the concentration of a specific carbon weight aromatic in the permeate is substantially proportional to the concentration of the same carbon weight aromatic in the feed. This is true across the carbon weight spectrum of the feed and permeate aromatics.
However, in some applications, certain carbon weight aromatic or carbon weight ranges of aromatics may be more beneficial as a product than mixed with other aromatics that are present in the feedstream. An example of this is in gasoline production where higher weight aromatics, for example C7 and above in the gasoline range material, generally have a higher octane value than the lower weight aromatics, such as C6 and below. Therefore, it would be beneficial in a membrane separation process if a shift in the average carbon weight of the aromatics in the permeate could be made with respect to the average carbon weight of the aromatics in the feed. Not only would such a process result in a higher value final product, but alternate methods of obtaining a similar product using conventional techniques would generally require higher installation and operating costs than if this segregation by carbon weight could be accomplished simultaneous with the aromatics/non-aromatics separation in the membrane separation process. The processes known in the art to make this secondary separation, such as a distillation step prior to or after the aromatic separation of the process by the membrane assembly, generally require more capital and energy costs than if a single-step, low-energy membrane separation process could provide a similar separation by selectively separating aromatics by carbon weight.